1. Field of the Invention
The present invention relates to a transmission for automobiles and, more particularly, a hydraulic transmission equipped with a lock-up clutch.
2. Description of the Prior Art
The torque converter which is now widely used for a hydraulic transmission for automobiles comprises an impeller connected to an engine output shaft, a turbine connected to an input shaft of a gear transmission mechanism and a stator adapted to deflect the flow of fluid which returns from said turbine to said impeller in order to effect a determined torque conversion between the engine output shaft and the input shaft of the gear transmission mechanism according to a difference between the rotational speed of said output shaft and said input shaft while transmitting rotational power by means of fluid dynamics. In this torque converter, there exists inevitably a slippage between the impeller and the turbine even under the optimum operating condition where the rotational speed of the turbine has almost approached the rotational speed of the impeller, thereby lowering the power transmission efficiency when compared with the direct transmission mechanism by a mechanical clutch. This results in an increase in the fuel consumption of the automobile, which leads to problems regarding economy of petroleum and exhaust gas purification. In order to solve these problems, a hydraulic transmission system equipped with a lock-up clutch has been proposed. This system includes in parallel a hydraulic transmission, for example, a torque converter and a friction clutch, the latter being adapted to be selectively actuated for effecting direct transmission according to the operational condition of the automobile.
A typical structure of the hydraulic transmission with a lock-up clutch is disclosed in U.S. Pat. No. 3338358. This transmission comprises an input member, an output member, a housing mounted to said input member, said housing defining a cylindrical chamber at the front portion thereof and providing an impeller at the rear portion thereof. In addition, a turbine is received in said housing and connected to said output member. A flexible disc radially traverses said cylindrical chamber and engages in a fluid-tight and slidable manner an inner wall of said cylindrical chamber thereby dividing said cylindrical chamber into a front first chamber and a rear second chamber. An annular clutch element extends radially inwardly from said housing in said second chamber, and a clutch disc is in driving connection with said output member, a peripheral portion of said clutch disc being interposed between said flexible disc and said annular clutch element. The second chamber and the torque converter provided therein are supplied with fluid from a fluid supply passage which opens in a portion of said second chamber. The fluid is exhausted from a fluid exhaust passage which opens in another portion of said second chamber after having traversed the chamber space. Thus normal operation of the torque converter is maintained, especially the operating temperature thereof, in the same manner as the conventional torque converter-type hydraulic transmission. On the other hand, said first chamber is selectively supplied with hydraulic pressure through a fluid passage which opens in a portion of said first chamber. The supply of hydraulic pressure is controlled by a control valve referred to as a lock-up shift valve. The lock-up shift valve is operated to supply hydraulic pressure to said first chamber when said input and output members are to be directly mechanically connected in by-passing the torque converter. When the hydraulic pressure has been supplied to said first chamber, the pressure in said first chamber becomes higher than that in said second chamber, whereby said flexible disc is biased from said first chamber toward said second chamber thereby compressing the periperhal portion of said clutch disc between the peripheral portion of the flexible disc and said annular clutch element. Thus, said input and output shafts are directly mechanically connected through a power transmission route extending from said housing to said clutch disc by way of said annular clutch element. The fluid transmission including a lock-up clutch as disclosed in U.S. Pat. 3,338,358 is considered to operate satisfactorily in that the power is basically transmitted by the fluid transmission and is optionally transmitted by the lock-up clutch according to operational conditions. However, since this structure is adapted so that the torque converter is constantly circulated with fluid, as in the conventional torque converter transmission incorporating no lock-up clutch, it requires two fluid passages for supplying and exhausting fluid into and out of said second chamber in which the torque converter is mounted. Furthermore, in order to bias said flexible disc to effect engagement of the lock-up clutch by opposing the fluid pressure (torque converter pressure) existing in said second chamber, the fluid pressure to be supplied to said first chamber must be higher than said torque converter pressure, thus requiring a complicated design for an oil pressure system related with the transmission.